The present invention generally relates to optical deflection elements and space optical matrix switching devices, and more particularly to an optical deflection element which uses the electrooptic effect to make an optical path switching and the like, and to a space optical matrix switching device such as a spatial light modulator and a parallel optical switch which are used in the fields of optical communications, optical information processing systems and the like.
FIG. 1 shows an example of a conventional space optical matrix switching device proposed in a Japanese Laid-Open Patent Application No. 58-111019. A diffraction grating (not shown) is formed on a photosensitive medium 1 using light interference, and light emitted from a photoemitter circuit 2 is passed through a collimator means 3. The diffraction grating on the photosensitive medium 1 is used to emit a light beam at a predetermined position on a photoreceptor circuit 4 so as to obtain an optical connection.
According to the space optical matrix switching device shown in FIG.1, a crystal such as BSO crystal having both photoconductive effect and electrooptic effect is used for the photosensitive medium 1. In this case, an arbitrary diffraction grating can be formed by the writing light and the diffraction grating becomes variable. As a result, it becomes possible to arbitrarily select the optical connection between the photoemitter circuit 2 and the photoreceptor circuit 4.
However, the space optical matrix switching device basically requires a write system for forming the diffraction grating and an optical system for separating the writing light and the signal light, and the device as a whole becomes bulky. In addition, the electrooptic effect of the electrooptic crystal such as the BSO crystal is generally small, and the diffraction efficiency of the grating induced by the light is small. Accordingly, when such a space optical matrix switching device is used for a conversion of an optical path, for example, the conversion becomes insufficient and the switching characteristic becomes poor.
On the other hand, optical deflection elements which use the acoustooptic effect or mechanically drives a prism or optical fiber have been proposed recently. However, it is difficult to realize a high-speed optical deflection by such optical deflection elements. Hence, there are proposals to make the optical deflection using the electrooptic effect.
A description will now be given of an example of a conventional optical deflection element which uses the electrooptic effect and proposed in a Japanese Laid-Open Patent Application No. 60-192926, by referring to FIGS. 2, 3A and 3B. An optical deflection element 11 includes two triangular column shaped electrooptic elements 12 and 13 which are bonded to form a parallelpiped shaped element 14. The electrooptic elements 12 and 13 have optic axes in mutually opposite directions. Optical fibers 15 and 16 which form optical paths are connected to a first surface of the electrooptic element 12, while optical fibers 17 and 18 which form optical paths are connected to a second surface of the electrooptic element 13, where the first and second surfaces are located on opposite ends of the element 14. An electrode layer 19 is formed on a top surface of the element 14 including a border part between the electrooptic elements 12 and 13, and an electrode layer 20 is formed on a bottom surface of the element 14 including the border part between the electrooptic elements 12 and 13. A driving power source 21 is connected across the electrode layers 19 and 20. Optic axes of the optical fibers 15 and 17 match, while the optical fibers 16 and 18 are respectively provided adjacent to the optical fibers 15 and 17.
As may be seen from FIG. 3A, when no voltage is applied across the electrode layers 19 and 20 by the driving power source 21, the light emitted from the optical fiber 15 into the element 14 progresses through the optical fiber 17 having the matching optical axis since the refractive indexes of the electrooptic elements 12 and 13 are approximately the same. On the other hand, when the voltage is applied across the electrode layers 19 and 20 by the driving power source 21, the light emitted from the optical fibers 15 and 16 into the element 14 is refracted at the boundary surface between the electrooptic elements 12 and 13 and progress through the respective optical fibers 18 and 17 because the refractive indexes of the electrooptic elements 12 and 13 having the optic axes in the mutually different directions change.
The optical deflection element 11 enables the switching of the optical path by turning the driving power source 21 ON/OF using the electrooptic effect. Hence, this optical deflection element 11 can be used as parts of the optical information processing systems and the like.
One of the parameters which determine the performance of the optical deflection element 11 is the resolution. A resolution N can be described by the following formula (1), where .phi. denotes a deflection angle and .theta. denotes a spread angle of the light beam with respect to the optical axis. EQU N=.phi./.theta. (1)
The performance of the optical deflection element 11 is better when the resolution N is larger. In addition, the spread angle .theta. of the light beam can be described by the following formula (2), where .lambda. denotes a wavelength of the light beam and .omega. denotes a beam diameter of the light beam. EQU .theta.=.epsilon..lambda./.omega. (2)
In the formula (2), .epsilon. is a constant which is dependent on the beam shape and the intensity distribution of the light beam, and this constant .epsilon. is close to one. For example, when the beam shape is circular and the intensity distribution is uniform, .epsilon.=1.22.
As may be readily seen from the formulas (1) and (2), it is necessary to reduce the spread angle .theta. of the light beam with respect to the deflection angle .phi. in order to improve the resolution N, and this may be achieved by increasing the beam diameter .omega..
In other words, in order to improve the resolution N of the optical deflection element 11, it is necessary to increase the thickness of the element 14 and enlarge the beam diameter .omega.. However, in this case, the refractive indexes of the electrooptic elements 12 and 13 decrease because the refractive indexes are proportional to the applied voltage and inversely proportional to the distance between the electrode layers 19 and 20. As a result, the capacity of the driving power source 21 must be increased in order to obtain the necessary deflection angle .phi., which is undesirable from the practical point of view.